The motion of a visible object can be readily recorded using a video camera, either in real time or using a high-speed or time-lapse technique. The resulting recording can be analysed frame by frame to extract numerical information about he motion. This technique is particularly effective if the motion is constrained to lie within a known plane, so that only two dimensional data need be captured.
The step of this process which presents most difficulty is the measurement of the object position in the video frame with sufficient accuracy to identify small perturbations in the motion. Three main techniques have been used to measure coordinates of points in video recordings. In a first technique, one or more measuring scales are incorporated in the original image, and the position is read using the scale(s). This method depends on the correct preparation of the subject and is not practicable when the subject is very large or very small. Fine scale graduations may be lost owing to the finite resolution of the video recording.
In a second prior technique, the position of the object is measured directly at the screen using a ruler. This method is laborious and subject to parallax error. Furthermore, the position of the image on the screen is somewhat variable because of jitter and line-tearing effects taking place on the display device.
In a third technique, the image is projected on to a drawing board and manual measurement is carried out using squared paper, a draughting machine or a simple ruler. This method is also laborious and parallax error is replaced by error due the relative movement of the projector and screen. In common with the second technique, jitter and line-tearing effects are also present.